Senescence is a major barrier to breast cancer progression and induced by a variety of stress signals including telomere erosion, DNA damage and oncogenic signaling. Oncogene-induced senescence (OIS) has been observed in human cancers and preclinical models, and is associated with inhibition of cancer initiation and progression. Moreover, senescence induction following chemotherapy has been correlated with favorable outcome in patients, also making activation of senescence an attractive target for therapeutic intervention. Recent studies in breast cancer have shown that induction of epithelial mesenchymal transition (EMT) can by-pass senescence and promote invasion and metastasis. However, there are gaps in our understanding the molecular genetics of senescence by-pass and the regulation of the equilibrium between senescence and EMT in breast cancer progression. Therefore, identifying mechanisms that regulate the switch between EMT and senescence is significant and will provide much needed insight into breast cancer progression and treatment. We have shown that Singleminded-2s (SIM2s; expressed from SIM2), a member of the bHLH/PAS family of transcription factors, is a tumor suppressor expressed in breast epithelial cells and down-regulated in transition from ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC) by inhibiting EMT and metastasis. Our recent results indicate that SIM2s is a potent inducer of senescence in breast cancer, and does so by interacting with ATM and p53, and is required for CDKN1A (p21WAF1) gene expression, a key component in senescence signaling. We therefore hypothesize that SIM2s is a tumor suppressor that blocks tumor formation by regulating the equilibrium between senescence and EMT. To test this hypothesis we propose two Specific Aims. In Aim 1, we will determine the effect of Sim2s gain of expression on ErbB2-mediated transformation of mouse mammary epithelial cells in vivo and the requirements of p21Waf1 and p53 in Sim2s-mediated senescence. In Aim 2, we will define the physical basis for, and functional outcomes of interactions between SIM2s and, ATM and p53 in the DDR pathway and induction of senescence in breast cancer cells. We will also investigate transcription factor recruitment and chromatin alterations associated with SIM2s-mediated CDKN1A gene expression. We anticipate that the proposed studies will better define the link between senescence and EMT as well as identify SIM2s as a novel target for breast cancer treatment.